In the creation of many electronic devices, such as semiconductor chips, and micro-mechanical devices films are deposited by physical vapour or chemical vapour deposition techniques and formation may be etched. Particularly where multiple devices are being formed on a single substrate, as, for example, happens when semiconductor chips are made from a single wafer, the uniformity of that deposition can become crucial if the rejection rate is not to reach uneconomic proportions. Although many designs and techniques have been developed to improve uniformity of deposition and etching, almost all techniques have a lack of uniformity, particularly between the centre of the substrate and the edge. This lack of uniformity has become more significant as wafer sizes have increased.